Demand defrost for heat pumps

ABSTRACT

A heat exchanger includes a motor that rotates a motor shaft to rotate a fan. An operating speed of the motor shaft determines an airflow through a heat exchanger coil. The heat exchanger also includes a sensor that detects a parameter of the motor shaft and a control. The parameter detected by the sensor is communicated to the control. The parameter is one of the operating speed of the motor shaft and a motor shaft torque. If the parameter is the operating speed, the control initiates a defrost cycle when the operating speed of the motor shaft is less than or equal to a threshold value. If the parameter is the motor shaft torque, the control initiates the defrost cycle when the torque application to the motor shaft is greater than or equal to a threshold value.

RELATED APPLICATION

This application claims priority to U.S. Provisional Patent Application No. 61/160,819, which was filed Mar. 17, 2009.

BACKGROUND OF THE INVENTION

During operation of a heat pump in a heating mode, frost can form on an outdoor coil. Determining when to defrost the heat pump is critical to efficiency. If a defrost cycle beings prematurely, energy can be unnecessarily wasted to heat the outdoor coil. If the defrost cycle is delayed too long, the performance of the heat pump can be reduced and cause the heat pump to operate less effectively. Changing weather conditions can also affect the rate at which moisture or frost accumulates on the outdoor coil.

Different parameters can be monitored to determine when it is desirable to initiate a defrost cycle. U.S. Pat. No. 3,726,104 discloses a non-motorized impeller is located in an airstream generated by a blower fan. A defrost signal is initiated when a condition of revolution of the impeller is detected. The impeller does not generate airflow, but is positioned within the airflow generated by the blower fan to be responsive to the airflow. United States Patent Application Number 2008/0098761 discloses a temperature sensor detects the temperature of the coil to determine when to begin a defrost cycle. U.S. Pat. No. 6,205,800 discloses a programmable controller compares an air temperature and a refrigerant temperature to calculate a difference. If the difference is greater than or equal to a defrost threshold, a defrost cycle is initiated. U.S. Pat. No. 6,318,095 discloses a control processor continuously monitors a difference between an outdoor coil temperature and an outdoor air temperature. When the difference exceeds a target value, the defrost cycle is initiated. Finally, United States Patent Application Number 2007/0180838 discloses a defrost cycle is initiated after a specific amount of time between defrost cycles, and the amount of time is continuously updated.

SUMMARY OF THE INVENTION

A heat exchanger includes a motor that rotates a motor shaft to rotate a fan. An operating speed of the motor shaft determines an airflow through a heat exchanger coil. The heat exchanger also includes a sensor that detects a parameter of the motor shaft and a control. The parameter is communicated to the control. The parameter is one of the operating speed of the motor shaft and a motor shaft torque. If the parameter is the operating speed, the control initiates a defrost cycle when the operating speed of the motor shaft is less than or equal to a threshold value. If the parameter is the motor shaft torque, the control initiates the defrost cycle when the torque application to the motor shaft is greater than or equal to a threshold value.

Another aspect of the invention provides a heat pump including a compressor for compressing a refrigerant, an indoor heat exchanger, an expansion device for expanding the refrigerant, and an outdoor heat exchanger. The outdoor heat exchanger includes a motor that rotates a motor shaft to rotate a fan, and an operating speed of the motor shaft determines an airflow through a heat exchanger coil of the outdoor heat exchanger. The outdoor heat exchanger includes a sensor that detects a parameter of the motor shaft and a control. The parameter is one of the operating speed of the motor shaft and a motor shaft torque. The parameter detected by the sensor is communicated to the control. When the parameter is the operating speed, the control initiates a defrost cycle when the operating speed of the motor shaft is less than or equal to a threshold value. When the parameter is the motor shaft torque, the control initiates the defrost cycle when the torque application to the motor shaft is greater than or equal to a threshold value.

Another aspect of the invention provides a method of determining when to initiate a defrost cycle. The method includes the steps of rotating a motor shaft to rotate a fan, and detecting a parameter of the motor shaft. The parameter is one of the operating speed of the motor shaft and the torque applied to the motor shaft. If the parameter is the operating speed of the motor shaft, the method further includes the step of initiating a defrost cycle if the operating speed is less than or equal to a threshold value. If the parameter is the torque applied to the motor shaft, the method further includes the step of initiating a defrost cycle if the torque applied to the motor shaft is greater than or equal to a threshold value.

These and other features of the present invention will be best understood from the following specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows:

FIG. 1 illustrates a refrigeration system operating in a cooling mode; and

FIG. 2 illustrates a refrigeration system operating in a heating mode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a refrigeration system 20 of a heat pump operating in a cooling mode. Refrigerant flows through the closed circuit refrigeration system 20. The refrigeration system 20 includes an indoor unit 30 located inside 32 a building and an outdoor unit 34 located outside 36 the building. In one example, the indoor unit 30 is a fan coil unit. The inside 32 of the building and the outside 36 of the building can be separated by a wall 38. In one example, the refrigeration system 20 is employed in a residence.

Refrigerant exits a compressor 22 in the outdoor unit 34 at a high pressure and a high enthalpy. A reversing valve 40 directs the refrigerant through a first heat exchanger 24, which operates as a condenser. The first heat exchanger 24 is an outdoor heat exchanger. In the first heat exchanger 24, the refrigerant flows through a coil 25 and rejects heat to air that is drawn over the coil 25 by a blower fan 42. The blower fan 42 is powered by an electronically commutated motor (ECM) 44. In the first heat exchanger 24, the refrigerant is condensed into a liquid that exits the first heat exchanger 24 at a low enthalpy and a high pressure.

The refrigerant bypasses an outdoor expansion device 26 b (described below) and travels to the indoor unit 30 through tubing 46. In the indoor unit 30, the cooled refrigerant then passes through an indoor expansion device 26 a, expanding the refrigerant to a low pressure. After expansion, the refrigerant flows through a second heat exchanger 28, which operates as an evaporator. The second heat exchanger is an indoor heat exchanger. A blower fan 31 draws air through the second heat exchanger 28 and over a coil 48. The refrigerant flowing through the coil 48 accepts heat from air, exiting the second heat exchanger 28 at a high enthalpy and a low pressure.

The refrigerant then flows back to the outdoor unit 34 through tubing 50. The refrigerant can flow through an accumulator 52, which regulates the amount of refrigerant flowing through the refrigeration system 20. The refrigerant then flows to the compressor 22, completing the cycle.

FIG. 2 illustrates the refrigeration system 20 operating in a heating mode. Refrigerant exits the compressor 22 in the outdoor unit 34 at a high pressure and a high enthalpy. The reversing valve 40 directs the refrigerant to the indoor unit 30 through the tubing 50.

In the indoor unit 30, the refrigerant flows through the second heat exchanger 28, which operates as a condenser. The blower fan 31 draws air through the second heat exchanger 28 and over the coil 48. In the second heat exchanger 28, the refrigerant flows through the coil 48 and rejects heat to air. The refrigerant is condensed into a liquid, exiting the second heat exchanger 28 at a low enthalpy and a high pressure. The refrigerant exits the coil 48 and bypasses the indoor expansion device 26 a.

The refrigerant exits the indoor unit 30 and flows through the tubing 46 towards the outdoor unit 34, where the refrigerant is expanded to a low pressure in the outdoor expansion device 26 b. After expansion, the refrigerant flows through the first heat exchanger 24, which operates as an evaporator. In the first heat exchanger 24, the refrigerant flows through the coil 25 and accepts heat from air that is drawn over the coil 25 by the blower fan 42, exiting the first heat exchanger 24 at a high enthalpy and a low pressure. The refrigerant can flow through the accumulator 52. The refrigerant then flows to the compressor 22, completing the cycle.

The outdoor unit 34 includes a housing 54 that contains the blower fan 42 that draws the air over the coil 25. An ECM motor 44 rotates a motor shaft 62 to rotate the blower fan 42.

Under normal operating conditions, the operating speed of the motor shaft 62 (or the revolutions per minute of the motor shaft 62) determines the airflow through the unrestricted coil 25. Microelectronics within the ECM motor 44 provide the ability to control the torque applied to the motor shaft 62 of the ECM motor 44 to maintain a constant torque on the motor shaft 62. Under normal conditions, the operating speed of the motor shaft 62 while operating with the desired shaft torque applied is known. When the refrigeration system 20 operates in the heating mode, frost may form on the coil 25 of the outdoor unit 34, which restricts airflow through the coil 25. When frost forms on the coil 25, the restriction of airflow also has the effect of increasing the pressure differential across the blower fan 42. Because the ECM motor 44 is controlled to produce constant shaft torque, the operating speed of the motor shaft 62 decreases until the load torque required by the blower fan 42 matches the shaft torque applied by the ECM motor 44. Maintaining the constant torque on the motor shaft 62 and a higher pressure differential across the blower fan 42 results in a new decreased motor shaft speed (i.e., lower revolutions per minute).

A sensor 64 detects the operating speed of the motor shaft 62 of the ECM motor 44 and communicates the detected operating speed to a control 66. A threshold value (a minimum revolutions per minute setpoint or a low shaft speed setpoint) is programmed into the control 66. The threshold value is specific for the coil 25 of the outdoor unit 34 and can be based on several factors, including the numbers of tubes in the coil 25, the size of the coil 25, the surface area of the coils 25, and the tonnage of the refrigeration system 20. As frost builds up on the coil 25, the operating speed of the motor shaft 62 decreases. The threshold value is the operating speed of the motor shaft 62 when the maximum airflow restriction through the coil 25 occurs due to frost buildup on the coil 25. When the sensor 64 detects that the operating speed of the motor shaft 62 is less than or equal to the threshold value, this indicates that the amount of frost built up on the coil 25 is the maximum amount allowed. The control 66 then initiates a defrost cycle to melt the frost on the coil 25. This allows for demand defrost. That is, the defrost cycle is initiated when the control 66 determines that a defrost cycle is necessary.

In another variation, under normal operating conditions, the operating speed of the motor shaft 62 (or the revolutions per minute of the motor shaft 62) determines the airflow through the unrestricted coil 25. Microelectronics within the ECM motor 44 provide the ability to control the speed of the ECM motor 44 to maintain a constant speed on the motor shaft 62. Under normal conditions, the torque applied to the motor shaft 62 while operating at the desired shaft speed is known. When the refrigeration system 20 operates in the heating mode, frost may form on the coil 25 of the outdoor unit 34, which restricts airflow through the coil 25 and can increase the torque load on the motor shaft 62. When frost forms on the coil 25, the restriction of airflow also has the effect of increasing the pressure differential across the blower fan 42. Because the ECM motor 44 is controlled to operate at a constant shaft speed and the pressure differential across the blower fan 42 has increased due to the frost build on the coil 25, the operating torque of the motor shaft 62 increases to match the increased torque load required by the blower fan 42 operating at the desired shaft speed.

A sensor 64 detects the torque applied to the motor shaft 62 of the ECM motor 44 and communicates the detected shaft torque to a control 66. A threshold value (a maximum torque setpoint) is programmed into the control 66. The threshold value is specific for the coil 25 of the outdoor unit 34 and can be based on several factors, including the numbers of tubes in the coil 25, the size of the coil 25, the surface area of the coils 25, and the tonnage of the refrigeration system 20. As frost builds up on the coil 25, the torque applied to the motor shaft 62 increases to maintain the required shaft speed. The threshold value is the operating torque of the motor shaft 62 when the maximum airflow restriction through the coil 25 occurs due to frost buildup on the coil 25. When the sensor 64 detects that the torque applied to the motor shaft 62 is greater than or equal to the threshold value, this indicates that the amount of frost built up on the coil 25 is the maximum amount allowed. The control 66 then initiates a defrost cycle to melt the frost on the coil 25. This allows for demand defrost. That is, the defrost cycle is initiated when the control 66 determines that a defrost cycle is necessary.

In one example, a defrost heater 68 is activated when the defrost cycle is initiated. The defrost heater 68 is positioned near the coil 25 and melts frost from the coil 25 when activated. When the defrost cycle is completed, the defrost heater 68 is deactivated. In another example, hot refrigerant from a discharge of the compressor 22 is directed to the first heat exchanger 24 along a line 70 to melt the frost. In this example, a valve 72 is located on the line 70 from an output of the compressor 22, and the control 66 opens the valve 72 to allow the hot refrigerant to flow to the coil 25. When the defrost cycle is completed, the control 66 closes the valve 72 to prevent the hot refrigerant from flowing to the coil 25. Although a defrost heater 68 and a valve 72 on a line 70 from the discharge of the compressor 22 are disclosed, any method of defrosting can be employed.

After the defrost cycle is completed, normal heating operation can resume. In one example, the defrost cycle ends after a predetermined amount of time. After the control 66 determines that the predetermined amount of time has passed, the control 66 sends a signal to deactivate the defrost heater 68 or close the valve 72. In another example, the defrost cycle ends after a temperature sensor 74 detects a predetermined temperature in the coil 25 and communicates this information to the control 66. When the predetermined temperature is reached, the control 66 sends a signal to deactivate the defrost heater 68 or close the valve 72. The predetermined temperature depends on the outdoor unit 34, and one skilled in the art would understand how to determine the predetermined temperature.

The initiation of the defrost cycle is time independent and only occurs on demand, so only the actual airflow restriction through the coil 25 will determine the need for a defrost cycle. This increases the operating efficiency of the heat pump by eliminating unnecessary defrost cycles and preventing heating operating when the airflow conditions through the coil 25 are not suitable.

The foregoing description is only exemplary of the principles of the invention. Many modifications and variations of the present invention are possible in light of the above teachings. The preferred embodiments of this invention have been disclosed, however, so that one of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For that reason the following claims should be studied to determine the true scope and content of this invention. 

1. A heat exchanger comprising: a motor that rotates a motor shaft to rotate a fan, wherein an operating speed of the motor shaft determines an airflow through a heat exchanger coil; a sensor that detects a parameter of the motor shaft; and a control, wherein the parameter is communicated to the control, wherein the parameter is one of the operating speed of the motor shaft and a motor shaft torque, wherein, when the parameter is the operating speed, the control initiates a defrost cycle when the operating speed is less than or equal to a threshold value, and wherein, when the parameter is the motor shaft torque, the control initiates the defrost cycle when the torque application to the motor shaft is greater than or equal to a threshold value.
 2. The heat exchanger as recited in claim 1 wherein the heat exchanger is an outdoor heat exchanger.
 3. The heat exchanger as recited in claim 1 wherein the threshold value is programmed in the control.
 4. The heat exchanger as recited in claim 1 wherein, when the parameter is the operating speed, the operating speed decreases as frost forms on the heat exchanger coil, and an amount of frost buildup is at a maximum allowed amount when the operating speed is less than or equal to the threshold value, and wherein, when the parameter is the torque applied to the motor shaft, the torque of the motor shaft increases as frost forms on the heat exchanger coil, and an amount of frost buildup is at a maximum allowed amount when the torque applied to the motor shaft is greater than or equal to the threshold value.
 5. The heat exchanger as recited in claim 1 wherein the defrost cycle ends after a predetermined amount of time.
 6. The heat exchanger as recited in claim 1 including a temperature sensor that detects a temperature in the heat exchanger coil, wherein the defrost cycle ends after the temperature sensor detects a predetermined temperature in the heat exchanger coil.
 7. The heat exchanger as recited in claim 1 including a defrost heater, wherein the defrost heater is activated when the defrost cycle is initiated.
 8. The heat exchanger as recited in claim 1 including a valve on a line from an output of a compressor, wherein the valve is opened to direct hot refrigerant from the compressor to the heat exchanger coil when the defrost cycle is initiated.
 9. A heat pump comprising: a compressor for compressing a refrigerant; an indoor heat exchanger; an expansion device for expanding the refrigerant; and an outdoor heat exchanger including a motor that rotates a motor shaft to rotate a fan, wherein an operating speed of the motor shaft determines an airflow through a heat exchanger coil of the outdoor heat exchanger, the outdoor heat exchanger including a sensor that detects a parameter of the motor shaft and a control, wherein the parameter is one of the operating speed of the motor shaft and a motor shaft torque; wherein the parameter is detected by the sensor and communicated to the control, wherein, when the parameter is the operating speed, the control initiates a defrost cycle when the operating speed is less than or equal to a threshold value, and wherein, when the parameter is the motor shaft torque, the control initiates the defrost cycle when the torque application to the motor shaft is greater than or equal to a threshold value.
 10. The heat pump as recited in claim 9 wherein the indoor heat exchanger is an evaporator and the outdoor heat exchanger is a condenser during a cooling mode, and the indoor heat exchanger is the condenser and the outdoor heat exchanger is the evaporator during a heating mode.
 11. The heat pump as recited in claim 9 wherein the threshold value is programmed in the control.
 12. The heat pump as recited in claim 9 wherein, when the parameter is the operating speed, the operating speed decreases as frost forms on the heat exchanger coil, and an amount of frost buildup is at a maximum allowed amount when the operating speed is less than or equal to the threshold value, and wherein, when the parameter is the torque applied to the motor shaft, the torque applied to the motor shaft increases as frost forms on the heat exchanger coil, and an amount of frost buildup is at a maximum allowed amount when the torque applied to the motor shaft is greater than or equal to the threshold value.
 13. The heat pump as recited in claim 9 including a temperature sensor that detects a temperature in the heat exchanger coil, wherein the defrost cycle ends after the temperature sensor detects a predetermined temperature in the heat exchanger coil.
 14. The heat pump as recited in claim 9 wherein the defrost cycle ends after a predetermined amount of time.
 15. The heat pump as recited in claim 9 including a defrost heater, wherein the defrost heater is activated when the defrost cycle is initiated.
 16. The heat pump as recited in claim 9 including a valve on a line from an output of the compressor, wherein the valve is opened to direct hot refrigerant from the compressor to the heat exchanger coil when the defrost cycle is initiated.
 17. A method of determining when to initiate a defrost cycle, the method comprising the steps of: rotating a motor shaft to rotate a fan, wherein an operating speed of the motor shaft determines an airflow through a heat exchanger coil; detecting a parameter of the motor shaft, wherein the parameter is one of the operating speed of the motor shaft and the torque applied to the motor shaft; and initiating a defrost cycle if an event occurs, and the event is one of the operating speed being less than or equal to a threshold value if the parameter is the operating speed, or the torque applied to the motor shaft being greater than or equal to a threshold value if the parameter is the torque applied to the motor shaft. 